The subject matter disclosed herein relates to a method and apparatus for controlling motion in a counterbalancing system and, more specifically, to a method and apparatus for determining a pre-torque direction during brake engagement and applying a pre-torque in the pre-torque direction prior to brake disengagement.
Elevators are typically implemented as counterbalancing systems. Accordingly, cables (or ropes) are often provided around a sheave (a grooved spindle or pulley), mounted to the top of an elevator cab in order to raise or lower the cab within an elevator shaft. A first end of the cables may be mounted to a first point at the top of the shaft and routed down and around a sheave mounted to the top of the cab. The cables may then be routed over a sheave mounted to an electric motor with a drive shaft there between. The cables may then continue around one or more sheaves mounted to a counterweight, then back to a second point at the top of the shaft. Use of the counterweight provides counterbalancing with the cab which may permit the motor to lift a differential weight between the counterweight and the cab (as opposed to the entire weight of the cab). Various configurations of cables, sheaves, and cable routing may be utilized.
To ease strain on the motor and provide further safety in the system, a brake is also typically used in conjunction with the motor. In such systems, when it is desired to remain stationary, the motor is typically enabled for zero speed, and while the motor is still enabled, the brake is enabled, and then the motor may be subsequently disabled while the brake is engaged to hold the sheave, cables, cab and counterweight stationary. Then, to resume motion, such as upon a call for the elevator to move to another floor, the motor is enabled for zero speed, then the brake is disengaged (or picked), then the motor continues to move the cab accordingly.
However, in certain circumstances which may depend on the system configurations and weights involved, the period between disengagement of the brake and activation of the motor may cause a brief rollback of the elevator cab (or of the counterweight). Moreover, this crossover period may also cause auditory noises and/or noticeable vibrations when the motor must quickly catch the car in an attempt to keep it stationary. These conditions may cause uneasiness for occupants of an elevator cab and/or disturbances for building occupants nearby and are therefore undesirable. What is needed is an improved counterbalancing system in which one or more of the aforementioned drawbacks may be minimized in such circumstances.